The present invention relates to a method of fabricating MIS transistors and, more particularly, to a method of fabricating MIS transistors having improved crystallinity by illuminating a high-speed ions to implant impurities into a semiconductor region and then improving the crystallinity by laser annealing, lamp annealing, or illumination of other equivalent intense light.
A structure comprising a semiconductor layer (S) on which a thin insulating film (I) and metal control electrodes (M) are formed is known as a MIS structure. A transistor having such a structure to control the electrical current flowing through the semiconductor layer is referred to as a MIS transistor. Where the insulating film is made of silicon oxide, the transistor is called a MOS transistor.
In the past, the activation step (for removing crystal defects produced on impurity implantation) conducted after the implantation of impurities into such MIS transistors has been carried out by thermal annealing. For this step, a high temperature as high as more than 1000xc2x0 C. is needed. In recent years, there is a demand for lower-temperature processes. Accordingly, alternatives to such high-temperature thermal annealing have been discussed. One promising method is to illuminate laser light or other intense light, for effecting activation. Depending on the used light source, this method is called laser annealing or lamp annealing.
A conventional method of fabricating MIS transistors, using laser annealing, is now described by referring to FIGS. 4(A)-4(E). An insulating film 402 is deposited as a base layer on a substrate 401. Then, a substantially intrinsic crystalline semiconductor film is deposited. This is photolithographically patterned into island-shaped semiconductor regions 403. Thereafter, an insulating film 404 acting as a gate-insulating film is deposited. Subsequently, gate electrodes 405 are deposited (FIG. 4(A)).
If necessary, the gate electrodes are anodized to form an anodic oxide 406 on the top and side surfaces of the gate electrodes and conductive interconnects. This method for forming such an anodic oxide and its merits are described in detail in Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary (FIG. 4(B)).
Then, an impurity is implanted by ion implantation or ion (plasma) doping. In particular, the substrate is placed in a fast stream of ions. Using the gate electrode portions, i.e., the gate electrodes and the surrounding anodic oxide, as a mask, an impurity is implanted into the island-shaped semiconductor regions 403 by a self-aligning process. In this way, doped regions 407 which will act as source and drain are formed (FIG. 4(C)).
Thereafter, intense light such as laser light is illuminated to recover the crystallinity which was deteriorated by the previous impurity implantation step (FIG. 4(D)).
An interlayer insulator 408 is then deposited, and contact holes are formed in it. Source and drain electrodes 409 are formed, thus completing MIS transistors (FIG. 4(E)).
In the method described above, when impurities are implanted, a large amount of impurities is introduced also into the gate-insulating film 404. These impurities themselves act as cores absorbing the laser light. In addition, defects produced by the impurity implantation absorb the laser light strongly. Especially, UV light is absorbed much, and light strong enough to activate the doped semiconductor regions 407 does not reach these regions. Usually, the insulating film is made of silicon oxide. The laser light is emitted from an excimer laser which has an excellent mass-producibility. If the silicon oxide is pure, it is sufficiently transparent to UV light emitted from an excimer laser. However, if impurities such as phosphorus and boron are present, the transparency deteriorates greatly. Hence, the activation is not sufficiently done.
If the doped regions are not sufficiently activated in this way, their resistivities are increased. It substantially follows that a resistor is inserted in series between source and drain. That is, the apparent mobility of the transistor drops. Also, the rising characteristics, or steepness, obtained when the transistor is turned on deteriorate.
In view of the foregoing problems, the present invention has been made. It is an object of the present invention to provide a method of efficiently carrying out an activation step using laser illumination.
If the thickness of the gate-insulating film described above is increased, then the breakdown voltage of the transistor is improved. However, this also requires that the accelerating voltage for the impurity ions be increased and that the implantation time be increased. Especially, where shallow doped regions are formed, a quite highly monochromatic ion beam is needed. In consequence, the dosage per unit time deteriorates severely.
On the other hand, where the gate-insulating film is removed and the semiconductor surface is exposed to efficiently conduct the implantation step, the surface is roughened when laser light or other intense radiation is illuminated and the doped regions are activated. As a result, the contact holes are deteriorated to an intolerable level.
It is another object of the invention to provide a method of efficiently carrying out an implantation step and a laser activation step.
In one embodiment of the present invention, an insulating film is formed as a gate-insulating film. Impurity ions are implanted into a semiconductor region through all or parts of the insulating film by irradiation of high-speed ions. Then, the insulating film is removed excluding the portions located under the gate electrode portions, thus exposing the semiconductor region. Laser light or other equivalent intense light is illuminated to perform an anneal. The above-described absorption of light by the insulating film does not take place. Activation can be done quite efficiently.
In another embodiment of the invention, a first insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the first insulating film is removed by a self-aligning process to expose the semiconductor surface. Then, impurity ions are implanted into the exposed semiconductor layer by irradiation of high-speed ions. Thereafter, a second insulating film of an appropriate thickness is formed on the exposed semiconductor layer. Thereafter, the semiconductor layer is irradiated with laser light or equivalent intense light through the second insulating film to perform an anneal. In this method, the above-described deterioration in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently.
In a further embodiment of the invention, a first insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the first insulating film is removed by a self-aligning process to expose the semiconductor layer surface. Then, a second insulating film of an appropriate thickness is formed on the semiconductor layer. Impurity ions are implanted into the semiconductor region through the second insulating film by irradiation of high-speed ions. Thereafter, the semiconductor layer is irradiated with laser light or other equivalent intense light to perform an anneal. In this method, the above-described decrease in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently.
In a still other embodiment of the invention, an insulating film is formed as a gate-insulating film. Using the gate electrode portions as a mask, the insulating film is etched to reduce its thickness to such an extent that ions of appropriate energy penetrate the film. Impurity ions are implanted into a semiconductor region by irradiation of high-speed ions through the thinned insulating film. Then, the semiconductor layer is irradiated with laser light or other equivalent intense light to perform an anneal. Prior to the laser irradiation, a transparent insulating film may be formed on the semiconductor layer surface. In this method, the above-described decrease in the implantation efficiency is prevented. Rather, an ion implantation step and a subsequent activation step can be accomplished quite efficiently. Other objects and features of the invention will appear in the course of the description thereof, which follows.